This invention relates to a controlled rectifier equipment with sinusoidal waveform supply current which converts AC voltage to DC voltage in such a manner that the waveform of the AC supply current may be a sinusoidal waveform.
In the conventional controlled rectifier equipment with sinusoidal waveform supply current, a DC current offset component is superposed on a supply current. The DC component causes an unnecessary electric power consumption and badly affects various electric power devices connected to the power supply. Therefore an electric current detector using a hall device (xe2x80x9cDCCTxe2x80x9d hereinafter) which can detect a DC current has been used to detect and eliminate the DC component.
The conventional controlled rectifier equipment with sinusoidal waveform supply current comprises a controlled rectifier circuit and a conduction control signal generating circuit. The conduction control signal generating circuit generates conduction control signals to semiconductor switching devices included in the controlled rectifier circuit so that a predetermined DC output voltage can be obtained and so that the waveform of the supply current may be sinusoidal waveform. A DCCT (an electric current detector for detecting direct current) and a reactor are connected in series between an AC current supply and a controlled rectifier circuit. The conduction control signal generating circuit obtains the difference between a DC current (including a DC offset component) detected by the DCCT and a current controlling signal which is needed or used to conform the output voltage of the controlled rectifier circuit to a predetermined DC voltage. Then a modulation circuit modulates the difference signal. The modulated difference signal is supplied to a drive circuit which output driving signals (conduction control signals) for driving the controlled rectifier circuit. The offset component detected by the DCCT is cancelled by the above difference operation, and as a result it is possible to input AC current having a sinusoidal waveform without the DC component.
However, a DCCT is expensive and a DC power supply is necessary for the DCCT as well as many accompanying circuits. Therefore it is increasingly difficult with conventional equipment to meet the recent cost decreasing demands.
It is an object of the present invention to provide a controlled rectifier equipment with sinusoidal waveform supply current which can be produced at a low cost without using an expensive electric current detector having hall devices.
It is another object of the present invention to provide a controlled rectifier equipment with sinusoidal waveform supply current which can eliminate a DC component using an AC current transformer.
It is still another object of the present invention to provide a controlled rectifier equipment with sinusoidal waveform supply current which can be manufactured in a simple configuration.
The present invention aims to improve a controlled rectifier equipment comprising a controlled rectifier circuit having a plurality of semiconductor switching devices which can be controlled, and a conduction control signal generating circuit. The conduction control signal generating circuit gives conduction control signals to the plural semiconductor switching devices for controlling the conducting angle of the plural semiconductor switching devices so that the controlled rectifier circuit may convert an AC power into an DC power and yet so that the waveform of the AC supply current maybe a sinusoidal waveform. The conduction control signal generating circuit includes an AC current detecting circuit, an AC voltage detecting circuit, a DC voltage detecting circuit, a current control signal generating circuit, a modulation circuit and a driving circuit. The AC current detecting circuit has an AC current detector for detecting the AC supply current and outputs an AC current detecting signal by processing an output signal form the AC current detector. The AC current transformer is used as the AC current detector. The AC voltage detecting circuit detecting an AC voltage supplied to the controlled rectifier circuit and generating an AC voltage detecting signal by processing the AC voltage. The DC voltage detecting circuit detecting a DC voltage outputted from the controlled rectifier circuit and generating a DC voltage detecting signal by processing the DC voltage. A current control signal generating circuit generates a current control signal based on the AC current detecting signal, the AC voltage detecting signal, the DC voltage detecting signal and a voltage command signal commanding a DC voltage value to be outputted from the controlled rectifier circuit. The current control signal is so formed as to make the waveform of the AC supply current a sinusoidal waveform and make the DC voltage from the controlled rectifier circuit such the DC voltage value as commanded by said DC voltage command signal. The modulation circuit for modulating the current control signal and generating a modulated current control signal. The driving circuit generates the conduction control signals based on the modulated current control signal. The current control signal generating circuit has a DC component eliminating circuit for eliminating a DC component which causes an offset of the AC supply current from the current control signal or the modulated current control signal.
A current control signal generating circuit comprises a DC voltage control system and a current control system. The DC voltage control system generates a current command signal based on the AC voltage detecting signal, the DC voltage detecting signal and the DC voltage command signal. The current command signal is used to make the DC voltage from the controlled rectifier circuit such the DC voltage value as commanded by the DC voltage command signal. The current control system generates a current control signal to make the output of the controlled rectifier circuit such a DC voltage value as commanded by the DC voltage command signal.
In the present invention an inexpensive AC current transformer (ACCT) is used as an AC current detector. However, it is not possible for an AC current transformer to detect whether or not a DC current offset component has been generated in the supply current to the controlled rectifier circuit. Therefore in the present invention a DC component eliminating circuit for eliminating a DC component which causes an offset of the AC current from the current control signal or the modulated current control signal. The DC component eliminating circuit may be arranged inside the current control system or between the modulation circuit and the drive circuit. In such an arrangement, it is possible to eliminate the DC current offset component which causes offsetting the AC current without using an expensive DCCT including expensive hall devices therein. In addition, it is possible to provide a controlled rectifier equipment with sinusoidal waveform supply current at a cost lower than that of conventional equipment and also to produce a controlled rectifier equipment in a simple configuration.
It is arbitrary how to form a DC component eliminating circuit. The DC component eliminating circuit may comprise a negative feedback circuit arranged across a signal line through which the current control signal or the modulated current control signal flows, and a low pass filter arranged in the negative feedback circuit. The low pass filter is easy to design and yet can be formed easily either by hardware or software, thus enabling the DC component eliminating circuit to be formed at a low cost. Therefore besides using an inexpensive AC current transformer, using a low pass filter which can be formed easily contributes to lowering the prices of the controlled rectifier equipment with sinusoidal waveform supply current. The low pass filter is constructed so as to cut off an AC power supply frequency component. In addition, the low pass filter extracts the DC component caused due to an offset generated primarily during signal processing in the AC current detecting circuit, the DC voltage detecting circuit, the AC voltage detecting circuit, the current control system, the DC voltage control system, the modulating circuit and the drive circuit.
It is arbitrary how to form a conduction control signal generating circuit. For example the DC voltage control system may comprise a first subtraction circuit, a voltage controller and a multiplier. The first subtraction circuit obtains the difference between said DC voltage command signal and the DC voltage detecting signal from the DC voltage detecting circuit, and outputs a difference signal. The voltage controller converts and amplifies said difference signal into AC output signal. The multiplier multiplies the AC voltage detecting signal from the AC voltage detecting circuit and the AC output signal from the voltage controller, and outputs the current control signal. The current control system comprises a second subtraction circuit and a current controller. The second subtraction circuit obtains the difference between the current control signal and the AC current detecting signal outputted by the AC current detecting circuit and outputs a difference signal. The current controller amplifies the difference signal outputted by the second subtraction circuit. In addition, the modulation circuit may comprise a PWM modulation circuit.
When a conduction control signal generating circuit has such an arrangement as described above, the DC component eliminating circuit may be arranged between the current controller and the PWM modulation circuit, or between the second subtraction circuit and the current controller, or between the PWM modulation circuit and the drive circuit. While any arrangement of the foregoing maybe used, designing each element becomes easier especially when the DC component eliminating circuit is arranged between the current controller and the PWN modulation circuit.